[llvm] [DebugInfo] Preserve line and column number when merging debug info. (PR #129960)

[Reid Kleckner via llvm-commits]

rnk wrote:

> > I think we will find additional opportunities and once we have a better idea, I can update the documentation and guidance at https://llvm.org/docs/HowToUpdateDebugInfo.html.
> > Does that sound reasonable to you?
> 
> I'd be inclined to figure out the principles here a bit earlier/somewhat up front, before we make changes. I'm especially concerned about a divergence between what's good for profilers and what's good for humans.

I think this is probably the most important thing: documenting the principles of what we do, why, what AFDO needs, what profilers need, etc, what modes we have, and what we expect them to do.

Over a long enough time horizon, it seems like we'd want to make this new behavior part of `-fdebug-info-for-profiling`, but I understand the need for separate feature flags in the short term.

https://github.com/llvm/llvm-project/pull/129960